953 research outputs found
Bayesian methods of astronomical source extraction
We present two new source extraction methods, based on Bayesian model
selection and using the Bayesian Information Criterion (BIC). The first is a
source detection filter, able to simultaneously detect point sources and
estimate the image background. The second is an advanced photometry technique,
which measures the flux, position (to sub-pixel accuracy), local background and
point spread function. We apply the source detection filter to simulated
Herschel-SPIRE data and show the filter's ability to both detect point sources
and also simultaneously estimate the image background. We use the photometry
method to analyse a simple simulated image containing a source of unknown flux,
position and point spread function; we not only accurately measure these
parameters, but also determine their uncertainties (using Markov-Chain Monte
Carlo sampling). The method also characterises the nature of the source
(distinguishing between a point source and extended source). We demonstrate the
effect of including additional prior knowledge. Prior knowledge of the point
spread function increase the precision of the flux measurement, while prior
knowledge of the background has onlya small impact. In the presence of higher
noise levels, we show that prior positional knowledge (such as might arise from
a strong detection in another waveband) allows us to accurately measure the
source flux even when the source is too faint to be detected directly. These
methods are incorporated in SUSSEXtractor, the source extraction pipeline for
the forthcoming Akari FIS far-infrared all-sky survey. They are also
implemented in a stand-alone, beta-version public tool that can be obtained at
http://astronomy.sussex.ac.uk/rss23/sourceMiner\_v0.1.2.0.tar.gzComment: Accepted for publication by ApJ (this version compiled used
emulateapj.cls
Microscopic surface structure of C/SiC composite mirrors for space cryogenic telescopes
We report on the microscopic surface structure of carbon-fiber-reinforced
silicon carbide (C/SiC) composite mirrors that have been improved for the Space
Infrared Telescope for Cosmology and Astrophysics (SPICA) and other cooled
telescopes. The C/SiC composite consists of carbon fiber, silicon carbide, and
residual silicon. Specific microscopic structures are found on the surface of
the bare C/SiC mirrors after polishing. These structures are considered to be
caused by the different hardness of those materials. The roughness obtained for
the bare mirrors is 20 nm rms for flat surfaces and 100 nm rms for curved
surfaces. It was confirmed that a SiSiC slurry coating is effective in reducing
the roughness to 2 nm rms. The scattering properties of the mirrors were
measured at room temperature and also at 95 K. No significant change was found
in the scattering properties through cooling, which suggests that the
microscopic surface structure is stable with changes in temperature down to
cryogenic values. The C/SiC mirror with the SiSiC slurry coating is a promising
candidate for the SPICA telescope.Comment: 25 pages, 6 figure
Detection of Optical Synchrotron Emission from the Radio Jet of 3C279
We report the detection of optical and ultraviolet emission from the
kiloparsec scale jet of the well-known quasar 3C~279. A bright knot, discovered
in archival V and U band {\it Hubble Space Telescope} Faint Object Camera
images, is coincident with a peak in the radio jet \sim0.6\arcsec from the
nucleus. The detection was also confirmed in Wide Field Planetary Camera-2
images. Archival Very Large Array and MERLIN radio data are also analyzed which
help to show that the high-energy optical/UV continuum, and spectrum, are
consistent with a synchrotron origin from the same population of relativistic
electrons responsible for the radio emission.Comment: 6 pages, 3 figs. accepted for publication in ApJL with minor
revision
Identification of new transitional disk candidates in Lupus with Herschel
New data from the Herschel Space Observatory are broadening our understanding
of the physics and evolution of the outer regions of protoplanetary disks in
star forming regions. In particular they prove to be useful to identify
transitional disk candidates. The goals of this work are to complement the
detections of disks and the identification of transitional disk candidates in
the Lupus clouds with data from the Herschel Gould Belt Survey. We extracted
photometry at 70, 100, 160, 250, 350 and 500 m of all spectroscopically
confirmed Class II members previously identified in the Lupus regions and
analyzed their updated spectral energy distributions. We have detected 34 young
disks in Lupus in at least one Herschel band, from an initial sample of 123
known members in the observed fields. Using the criteria defined in Ribas et
al. (2013) we have identified five transitional disk candidates in the region.
Three of them are new to the literature. Their PACS-70 m fluxes are
systematically higher than those of normal T Tauri stars in the same
associations, as already found in T Cha and in the transitional disks in the
Chamaeleon molecular cloud. Herschel efficiently complements mid-infrared
surveys for identifying transitional disk candidates and confirms that these
objects seem to have substantially different outer disks than the T Tauri stars
in the same molecular clouds.Comment: Accepted for publication in A&A. 16 pages, 9 figures, 7 table
The ASTRO-F Mission : Large Area Infrared Survey
ASTRO-F is the first Japanese satellite mission dedicated for large area
surveys in the infrared. The 69cm aperture telescope and scientific instruments
are cooled to 6K by liquid Helium and mechanical coolers. During the expected
mission life of more than 500 days, ASTRO-F will make the most advanced all-sky
survey in the mid- to far-infrared since the Infrared astronomical Satellite
(IRAS). The survey will be made in 6 wavebands and will include the first all
sky survey at >100-160(mu)m. Deep imaging and spectroscopic surveys with
pointed observations will also be carried out in 13 wavelength bands from
2-160(mu)m. ASTRO-F should detect more than a half million galaxies tracing the
large-scale structure of the Universe out to redshifts of unity, detecting
rare, exotic extraordinarily luminous objects at high redshift, numerous brown
dwarfs, Vega-like stars, protostars, and will reveal the large-scale structure
of nearby galactic star forming regions. ASTRO-F is a perfect complement to
Spitzer Space Telescope in respect of its wide sky and wavelength coverage.
Approximately 30 percent of pointed observations will be allocated to an
open-time opportunity. Updated pre-flight ensitivities as well as the
observation plan including the large-area surveys are described.Comment: accepted for publication in Advances in Space Research, 15 pages, 7
Postscript figures, uses elsart.cl
SPICA: the next generation Infrared Space Telescope
We present an overview of SPICA, the Space Infrared Telescope for Cosmology
and Astrophysics, a world-class space observatory optimized for mid- and far-IR
astronomy (from 5 to ~210um) with a cryogenically cooled ~3.2m telescope (<6
K). Its high spatial resolution and unprecedented sensitivity in both
photometry and spectroscopy modes will enable us to address a number of key
problems in astronomy. SPICA's large, cold aperture will provide a two order of
magnitude sensitivity advantage over current far-IR facilities (lambda>30 um
wavelength). In the present design, SPICA will carry mid-IR camera,
spectrometers and coronagraph (by JAXA institutes) and a far-IR imager
FTS-spectrometer, SAFARI (~34-210 um, provided by an European/Canadian
consortium lead by SRON). Complementary instruments such as a far-IR/submm
spectrometer (proposed by NASA) are also being discussed. SPICA will be the
only space observatory of its era to bridge the far-IR wavelength gap between
JWST and ALMA, and carry out unique science not achievable at visible or submm
wavelengths. In this contribution we summarize some of the scientific advances
that will be made possible by the large increase in sensitivity compared to
previous infrared space missions.Comment: 6 pages; to appear in the proceedings of the conference: "Conditions
and impact of star formation: New results with Herschel and beyond" (The 5th
Zermatt ISM Symposium
Infrared study of transitional disks in Ophiuchus with Herschel
Context. Observations of nearby star-forming regions with the Herschel Space
Observatory complement our view of the protoplanetary disks in Ophiuchus with
information about the outer disks. Aims. The main goal of this project is to
provide new far-infrared fluxes for the known disks in the core region of
Ophiuchus and to identify potential transitional disks using data from
Herschel. Methods. We obtained PACS and SPIRE photometry of previously
spectroscopically confirmed young stellar objects (YSO) in the region and
analysed their spectral energy distributions. Results. From an initial sample
of 261 objects with spectral types in Ophiuchus, we detect 49 disks in at least
one Herschel band. We provide new far-infrared fluxes for these objects. One of
them is clearly a new transitional disk candidate. Conclusions. The data from
Herschel Space Observatory provides fluxes that complement previous infrared
data and that we use to identify a new transitional disk candidate.Comment: 21 pages, with 5 figures. Accepted in Astronomy & Astrophysic
Feasibility and performances of compressed-sensing and sparse map-making with Herschel/PACS data
The Herschel Space Observatory of ESA was launched in May 2009 and is in
operation since. From its distant orbit around L2 it needs to transmit a huge
quantity of information through a very limited bandwidth. This is especially
true for the PACS imaging camera which needs to compress its data far more than
what can be achieved with lossless compression. This is currently solved by
including lossy averaging and rounding steps on board. Recently, a new theory
called compressed-sensing emerged from the statistics community. This theory
makes use of the sparsity of natural (or astrophysical) images to optimize the
acquisition scheme of the data needed to estimate those images. Thus, it can
lead to high compression factors.
A previous article by Bobin et al. (2008) showed how the new theory could be
applied to simulated Herschel/PACS data to solve the compression requirement of
the instrument. In this article, we show that compressed-sensing theory can
indeed be successfully applied to actual Herschel/PACS data and give
significant improvements over the standard pipeline. In order to fully use the
redundancy present in the data, we perform full sky map estimation and
decompression at the same time, which cannot be done in most other compression
methods. We also demonstrate that the various artifacts affecting the data
(pink noise, glitches, whose behavior is a priori not well compatible with
compressed-sensing) can be handled as well in this new framework. Finally, we
make a comparison between the methods from the compressed-sensing scheme and
data acquired with the standard compression scheme. We discuss improvements
that can be made on ground for the creation of sky maps from the data.Comment: 11 pages, 6 figures, 5 tables, peer-reviewed articl
GREAT: the SOFIA high-frequency heterodyne instrument
We describe the design and construction of GREAT, the German REceiver for
Astronomy at Terahertz frequencies operated on the Stratospheric Observatory
for Infrared Astronomy (SOFIA). GREAT is a modular dual-color heterodyne
instrument for highresolution far-infrared (FIR) spectroscopy. Selected for
SOFIA's Early Science demonstration, the instrument has successfully performed
three Short and more than a dozen Basic Science flights since first light was
recorded on its April 1, 2011 commissioning flight.
We report on the in-flight performance and operation of the receiver that -
in various flight configurations, with three different detector channels -
observed in several science-defined frequency windows between 1.25 and 2.5 THz.
The receiver optics was verified to be diffraction-limited as designed, with
nominal efficiencies; receiver sensitivities are state-of-the-art, with
excellent system stability. The modular design allows for the continuous
integration of latest technologies; we briefly discuss additional channels
under development and ongoing improvements for Cycle 1 observations.
GREAT is a principal investigator instrument, developed by a consortium of
four German research institutes, available to the SOFIA users on a
collaborative basis
The dusty environment of HD 97300 as seen by Herschel and Spitzer
Aims. We analyze the surroundings of HD 97300, one of two intermediate-mass
stars in the Chamaeleon I star-forming region. The star is known to be
surrounded by a conspicuous ring of polycyclic aromatic hydrocarbons (PAHs).
Methods. We present infrared images taken with Herschel and Spitzer using 11
different broad-band filters between 3.6 um and 500 um. We compare the
morphology of the emission using cuts along different position angles. We
construct spectral energy distributions, which we compare to different dust
models, and calculate dust temperatures. We also derive opacity maps and
analyze the density structure of the environment of HD 97300.
Results. We find that HD 97300 has no infrared excess at or below 24 um,
confirming its zero-age main-sequence nature. The morphology of the ring is
very similar between 3.6 um and 24 um. The emission at these wavelengths is
dominated by either PAH features or PAH continuum. At longer wavelengths, only
the northwestern part of the ring is visible. A fit to the 100-500 um
observations suggests that the emission is due to relatively warm (~26 K) dust.
The temperature gradually decreases with increasing distance from the ring. We
find a general decrease in the density from north to south, and an approximate
10% density increase in the northeastern part of the ring.
Conclusions. Our results are consistent with the theory that the ring around
HD 97300 is essentially a bubble blown into the surrounding interstellar matter
and heated by the star.Comment: 6 pages, 3 figures, accepted for publication in A&
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